Typically, work vehicles, such as tractors and other agricultural vehicles, have a mechanical transmission such as a continuously variable transmission (CVT) or a power shift transmission (PST). Conventionally, each type of transmission utilizes a parking brake in order to maintain the vehicle in a parked or stopped condition when the vehicle is placed in park or when performing a hill holding operation. In addition, the parking brake may also be used to reduce the speed of the work vehicle, such as when the work vehicle experiences a loss of functionality (e.g., loss of electrical power and/or hydraulic pressure).
In many instances, conventional parking brakes for work vehicle transmissions are configured to be engaged and disengaged via an electric actuator (e.g., a solenoid activated actuator). Unfortunately, several disadvantages are typically associated with the use of electric actuators for engaging and disengaging parking brakes. Specifically, the electric actuators typically present mounting challenges within the work vehicle, since the actuator is often mounted at a location that is significantly spaced apart from the transmission. In addition, operation of an electric actuator requires the use of complex computer logic to ensure that the associated parking brake is engaged and disengaged at the appropriate time. However, even despite the use of complex computer logic, system faults (e.g., short circuits, etc.) may often result in the electric actuator being unintentionally commanded to engage the parking brake, which can cause significant damage to the brake itself or to the transmission as a whole when the vehicle is still in motion.
More recently, hydraulic brake actuators have been utilized to engage and disengage parking brakes of work vehicle transmissions. Typically, a piston of a hydraulic actuator is moved linearly due to selective increases and decreases in hydraulic fluid within a housing of the hydraulic actuator. A rod connected to the piston is also moved linearly, and this rod is connected via a suitable mechanical linkage to the parking brake. The parking brake is selectively engaged and disengaged as a result of the movement of the piston.
In many cases, a decrease in the hydraulic fluid within a chamber of the housing causes engagement of the parking brake. This allows the parking brake to act as an automatic emergency brake if hydraulic fluid pressure is lost. A loss in such pressure will result in a decrease in the hydraulic fluid within the chamber and a corresponding engagement of the parking brake. However, this automatic emergency brake functionality has a number of disadvantages. For example, once the parking brake has been engaged in cases wherein hydraulic fluid pressure is lost, it cannot be disengaged without repair to the hydraulic fluid supply or use of, for example, an independent pump to re-pressurize the line. Further, engagement of the parking brake in such situations is immediate, resulting in immediate and rapid deceleration of the work vehicle. This can cause injury to the user of the work vehicle and damage to the work vehicle itself.
Accordingly, improved parking brake systems are desired in the art. In particular, parking brake systems which allow for selective engagement and disengagement in emergency situations, and which allow for brake modulation during braking events, would be advantageous.